The present invention relates generally to container closures, and more particularly to closures for use in containers that may experience internal pressure changes once sealed such as, for example, hot-fill containers and containers subject to pasteurization processes.
The background of the present invention will be described in connection with closures for hot-fill applications. It should be understood, however, that the use of the closure of the present invention has wider applicability and can be employed on any type of container.
Internally threaded, plastic cap closures have found widespread application for use in connection with hot-fill plastic containers by virtue of their low manufacturing costs and sealing performance. In a conventional hot-fill process, a hot beverage product is introduced into the plastic container, typically filling most of the container. The fluid is heated during a pasteurization or sterilization process to remove bacteria or other contamination. The plastic container is hermetically sealed with a cap while the product is still hot. Since the beverage product is typically not filled to the top of the container, a headspace of air is provided between the liquid enclosed within the plastic container and an inner surface of the cap. The temperature of the liquid varies from a high of about 205° F., the typical hot-fill temperature, to about 40° F., the typical refrigeration temperature. A change in temperature, from hot to cold, decreases the internal pressure of the sealed container and creates a vacuum within the container primarily as a result of the thermal contraction of the liquid in the container. This decrease in pressure can distort and/or deform the geometry of the container if the container cannot structurally support the pressure difference between the external ambient pressure and the lower internal pressure of the container. Deformation of the container generally pushes the fluid upwardly and decreases the headspace volume. For example, for a typical 16-ounce container, thermal contraction equates to roughly 3% of the total liquid volume, or 0.9 cubic inches when the stored contents are cooled from about 185° F. to about 40° F.
Current containers are engineered to collapse at specific locations or are reinforced with vacuum panels and/or flexible bases to compensate for the vacuum. Vacuum-reactive mechanisms are very efficient to maintain a balanced pressure and keep the remaining structural geometry of the container from collapsing. Further, labeling of the container is difficult because containers employing raised and/or recessed vacuum panels possess reduced surface area. The reduction of surface area also restricts the ornamental design of the label, restricts the placement of the label, and often leads to unattractive wrinkling of the label.
There have been attempts to prevent container deformation by designing plastic closures that will compensate for the vacuum created by the cooling of a hot-filled liquid. For example, U.S. Pat. No. 7,621,412 discloses a cap that includes an air permeable membrane covering a through-hole in the cap to permit pressure equalization between the interior of the container and the ambient atmosphere during cooling of the container's contents. This design, however, allows air to be pulled directly into the product and requires the membrane be plugged to seal the contents of the container from further ingress or egress of fluids. U.S. patent application Publication No. 2007/0228058 discloses an expandable plastic closure that flexes in response to pressure. This closure includes a series of elevated substantially flat concentric panels of varying diameters. This design, however, potentially allows for uneven top surfaces of the sealed cooled containers. Finally, U.S. patent application Publication No. 2009/0179032 discloses a plastic closure having an expandable bellows that extend within the neck of the closure. During attachment of such closure to the neck of the container, the bellows is compressed to force air positioned therein into the container which creates a pressure increase within the container. The pressure increase is sufficiently large such that when the container is cooled, a pressure decrease sufficient enough to distort the container allegedly will not form. A disadvantage of this design is that there are multiple components that are susceptible to contamination behind the compressed liner/bellows and the disclosed configuration would not be readily adaptable to a pasteurization process where internal pressure would be increased.
Accordingly, there is a need in the art for a plastic closure that will significantly reduce or prevent container deformation by compensating for the vacuum created by the liquid hot-fill/subsequent cooling process without suffering from the above-mentioned drawbacks.